1. Field of the Invention
The invention relates to a method for inhibiting potato tuber sprouting.
2. Description of the Prior Art
Typically, tubers are harvested, allowed to suberize (i.e., allow the "skin" or periderm layer to toughen) at warm temperatures for about 10 days, then gradually cooled down to the storage temperature of about 10.degree. C. For the first 1-2 months after harvest, the tubers remain dormant and exhibit little inclination to sprout. However, after this period the tubers must be chemically treated to prevent sprouting from occurring, which sprouting causes numerous deleterious effects to the tubers. These include a loss of fresh weight, the conversion of starch to sugars, and a decrease in the quality and appearance of tubers sold fresh. Sprouts and the surrounding tissue also contain elevated levels of toxic glycoalkaloids, which are not destroyed by cooking.
Chlorpropham (CIPC; 1-methylethyl-3-chlorophenylcarbamate) is currently used to control sprouting throughout the industry. Although CIPC has been used effectively for over three decades, questions concerning its toxicology have been raised, and it is currently under review by the Environmental Protection Agency. CIPC is known to be among the three pesticides found in the highest concentrations in the diet of the average American [Gartrell et al., J. Assoc. Off. Anal. Chem., 69:146-159 (1986)] and comprises over 90% of the total synthetic residues found in U.S. potatoes [Klocke et al., J. Chem. Ecol., 13:2131-2141 (1987)]. Therefore, a pressing needs exists to find other, more environmentally acceptable sprout inhibitors for tubers.
Currently, several research groups in the United States and Europe are investigating alternative chemical inhibitors [Rama and Narasimham, J Food Sci Technol., 24:40-42 (1987)].
For many centuries, the Incas of South America and their descendants have buried potato tubers in pits that are layered with soil and the leaves of Muna plants that belong to the mint family Lamiaceae, and the genera Minthostachys and Satureja. This treatment prevents sprouting and excessive fresh weight loss, and repels insect pests. These Muna plants contain copious amounts of essential oils that are substantially comprised of monoterpenes. Aliaga and Feldheim [Ernahrung, 9:254-256 (1985)] and Feldheim ["Practicability and Mode of Action of Quality Storage of Potatoes After Harvest," In Report of a Lecture Given to the German Institute for Quality Research (Plant Nutrition Products), March 1985, 6 pages] reported that the oil from the Muna plants was more effective than CIPC in inhibiting sprouting, fresh weight loss, and the incidence of rotted tuber parts over a period of 120 days. The authors also reported that the main components of the oil, including the monoterpenes .alpha.- and .beta.-pinene and limonene, and the oxygenated monoterpenes pulegone and menthone/isomenthone, are effective in this regard.
Various aromatic acids, aldehydes, phenols and their derivatives are known to be phytotoxic [Putnam and Tang, "Allelopathy: State of the Science," in Putnam and Tang (ed.), The Science of Allelopathy, John Wiley & Sons, N.Y., 1986, pages 1-19]. In particular, cinnamic and benzoic acid derivatives have been frequently reported as being involved in allelopathy among plants [Putnam and Tang, ibid.]. These compounds are derived from the amino acids phenylalanine and tyrosine via the shikimic acid pathway and may be released from plant residues at phytotoxic levels [Mandava, "Chemistry and Biology of Allelopathic Agents," in Thompson (ed.), The Chemistry of Allelopathy, American Chemical Society, Washington, D.C., 1985, pages 33-54].
Volatile aromatic compounds are commonly present in plant essential oils, and many of these are used in substantial quantities as flavorings and in perfumes [Leung, in Encyclopedia of Common Natural Ingredients Used in Foods, Drugs, and Cosmetics, Johny Wiley & Sons, N.Y., 1980, page 409]. Benzaldehyde, salicylaldehyde and substituted benzoic acids have been found in uncooked and baked potato tubers [Coleman et al., J. Agric. Food Chem., 29:42-49 (1981); Mazza and Pietrzak, Food Chem., 36:97-112 (1990); and Nursten and Sheen, J. Sci. Food Agric., 25:643-663 (1974)]. Several of these compounds have been shown to be inhibitory to the growth of plants, fungi, and bacteria [Farag et al., J. Food Sci., 54:74-76 (1980); Hitokoto et al., Appl. Environ. Microbiol., 39:818-822 (1980); Kurita et al., Agric. Biol. Chem., 45:945-952 (1981); Pauli and Knobloch, Z. Lebensm. Unters. Forsch., 185:10-13 (1987); Powell and Spencer, "Phytochemical Inhibitors of Velvetleaf (Abutilon threoprasti) Germination as Models for New Biorational Herbicides," in Cutler (ed.), Biologically Active Natural Products: Potential Use in Agriculture, ACS Symposium Series No. 380, American Chemical Society, Washington, D.C. (1988), pages 211-232; and Putnam and Tang, ibid.].